The folding of a protein from a sequence of amino acids to a well-defined tertiary structure is one of the most studied and enigmatic events to take place in biological systems. Relatively recently, it has been established that some proteins and peptides are able to take on conformations other than their native fold to form long fibres known as amyloid. In vivo, these are associated with misfolding diseases, such as Alzheimer's disease, Type 2 diabetes and the amyloidoses. In vitro, peptide assembly leads to amyloid-like fibres that have high stability, resistance to degradation and high tensile strength. Remarkably, despite the lack of any obvious sequence similarity between these fibrillogenic proteins and peptides, all amyloid fibrils share common structural characteristics and their underlying structure is known as ‘cross-β’. Nature is rich in β-sheet protein assemblies such as spider silk and other ‘useful’ amyloids such as curli from Escherichia coli, where the strength of fibrils is fundamental to their function.